I doubt you will notice too much. The Fluance center is so bad that anything is better. At worst the center will outshine the fronts...but your receiver will help alleviate the difference...if it has to.
... I was worried that I could do damage to my receiver, people on here imply that mixing and matching speakers is a cardinal sin.
As long as the ohm rating is something your receiver can handle it won't hurt your system, even if the resistance doesn't match the others.
The reason people don't want a mismatched center is that you will often hear the difference. The front speakers should be timbre matched so when the sound pans from side to side you don't get a change in audio. Basically the same sound coming from the different speakers will sound different.